1. Field of the Invention
This invention relates to crane control systems in general, and specifically to a synchronization system for level-beam, cantilever and overhead gantry cranes having a hoist suspended from a trolley for lifting a load and a trolley for transporting the load laterally along one or more beams associated with the crane.
2. Description of the Background
Level-beam, cantilever cranes and overhead gantry cranes such as Rail-Mounted Gantry cranes (“RMG”) and Rubber Tire Gantry cranes (“RTG”), are used to move loads of varying size and weight from one location to another. Often cranes such as the RTG crane shown in FIG. 1 include one or more trolleys and hoists, which are used to move large, heavy loads. Due to a variety of factors such as uneven load weight, wind, crane motion, and the acceleration and deceleration of the trolley, loads tend to sway or swing during movement. Load sway is problematic because loading and unloading operations cannot take place if the load is swaying at the end of movement. If a load is swaying at the end of movement, an operator must either wait for the load to stop swaying or maneuver the trolley and/or hoists in a manner that negates the swaying movement. This waiting and/or maneuvering can take up to one third or more of the total transfer time.
Several anti-sway systems have been developed to counteract the sway of loads during movement. One such system is disclosed in Overton, U.S. Pat. No. 5,526,946. The anti-system system disclosed in Overton uses a double-pulse, anti-sway algorithm that is based on a single pendulum length to negate the affects of sway caused by acceleration of the trolley, movement of the hoists, and external factors.
However, not all sway movement is in the form of a single pendulum as shown in FIGS. 2 and 2A. Often time uneven hoists or misaligned trolleys cause sway that has a circular motion, which is difficult to control, rather than a single pendulum-type motion. For example, FIGS. 3 and 3A show an example of one type of a circular sway caused by misaligned front and rear trolleys. In this example, all the hoists are even, i.e., at the same height, but the front and rear trolleys are misaligned, i.e., the front and rear trolleys are not square with the beams of the crane. Likewise, FIGS. 4 and 4A illustrate an example of a circular sway that is caused by uneven hoists. Here, the trolleys are properly aligned but the hoists are not even.
To address the problem of uneven hoists and misaligned trolleys, an operator must skillfully synchronize all the hoists and trolleys using multiple independent controls, which is time consuming and imperfect. Other methods for control require mechanical bridges that replace or are connected to the trolleys and hoists in order to mechanically synchronize them. Such devices are very expensive, and therefore not practical to implement.
Given the limitations of the prior art, there exists a need for a single control for all trolleys and a single control for all hoists so that synchronization of the trolleys and hoists can be obtained quickly and efficiently. By synchronizing the trolleys and hoists, uncontrollable swing of the lifted load will be greatly reduced, thereby improving productivity, increasing safety, and reducing operator fatigue.
It would also be an improvement in the art to enable synchronization of the trolleys and hoists so that anti-sway technology can be used to eliminate further load sway during lateral movement of the load.